1. Field of the Invention
The present invention relates to a ferrous alloy formed with an Fe--Cr stainless steel as a substrate and an Fe--Al diffusion layer of improved hardness, which may be used for sliding parts such as gears and bearings or blades of electric shavers or hair clippers, and a method of making the ferrous alloy.
2. Disclosure of the Prior Art
In the past, carbon tool steels, high carbon stainless steels, and precipitation-hardening stainless steels have been used for sliding parts such as gears and bearings or cutting tools. Those steels exhibit excellent mechanical toughness and shockproof, although, the surface hardness and wear resistance of the steels are not always enough to provide the sliding parts or cutting tools having an extended service life. To improve this problem, it is proposed to use ceramic materials such as aluminum oxide (Al2O3) or zirconium oxide (ZrO2) of excellent hardness and wear resistance. However, there is another problem that the mechanical toughness of the ceramic materials is much lower than that of the steels. In addition, it is not easy to machine the ceramic materials to various shapes of the sliding parts or cutting tools. Therefore, it is desired to develop a material having improved hardness and wear resistance, while keeping the mechanical strength and toughness of the steels.
For example, Japanese Patent Early Publication [KOKAI] No. 4-250995 teaches a blade material for electric shavers and a method of making the same. The blade material comprises a substrate such as a high hardness and nonmagnetic stainless steel, Fe--Mn alloy or Be--Cu alloy, an intermetallic compound layer of Al and metal elements contained in the substrate, e.g., Ni and Fe, and an Al2O3 layer on the intermetallic compound layer. The blade material can be made by the steps of cladding Ni and Al foils to the substrate such that the Ni foil is positioned between the substrate and Al foil, heating the cladded substrate in vacuum or oxidation atmosphere to form an intermetallic compound layer of NiAl and/or Ni3Al and the Al2O3 layer. When the heat-treatment is performed in vacuum, the cladded substrate is heated at a temperature of 400.degree. C. to 650.degree. C. for 1 to 20 minutes. When the heat-treatment is performed in the oxidation atmosphere, the cladded substrate is heated at a temperature of 600.degree. C. to 1000.degree. C. for 5 to 20 hours. However, since the diffusion rate of Ni atoms into the substrate is much slower than that of Al atoms into the substrate, and also Ni atoms prevent the diffusion of Al atoms into the substrate, there is a problem that the adhesion between the substrate and intermetallic compound layer is not enough.
UK Patent No. 1278085 teaches an aluminum diffusion-coated steel having sulphidation resisting properties in an atmosphere of high temperature and pressure. The coated steel comprises a steel having a surface layer formed by an aluminum diffusion-coating method. The method is characterized by a heat-treatment at a temperature of 800 to 950.degree. C. The surface layer consists of aluminum alloy, and has a thickness not exceeding 300 .mu.m. The Al content of the surface region of the surface layer is less than 30 wt %. For example, the substrate is an alloy steel containing carbon not exceeding 0.5 wt % and at least one element selected from Mo in the amount of 0.1 to 1.2 wt %, Cr not exceeding 10 wt % and Ni not exceeding 4.5 wt %. For the aluminum diffusion-coating method, a powder pack method, gas method, ceramic adsorption method or hot-dip and diffusing method is applicable. However, since the Al content of the surface layer is less than 30 wt %, it is difficult to form hard Al--Fe intermetallic compounds such as Al13Fe, Al13Fe4 or Al5Fe2 in the surface layer. As a result, the surface layer would be not sufficient to provide high hardness and wear resistance.